I have a '66 Mustang with Granada disc brakes on all 4 corners.  I purchased an aftermarket Wilwood proportioning valve to plumb into the rear brake line.  I have a question about the proportioning valve part of the discussion.  For option 3  on Mustang Steve's website, it lists: Use a Granada proportioning valve in conjunction WITH an aftermarket adjustable proportioning valve.  In order to make it work properly, the proportioning function must be removed from the Granada valve.  To do so, the spring and inner piston need to be removed.  Take the big nut/plug off. (On the right in the picture below) remove the spring and piston (about 1-1/4" long) and put the big nut/plug back on.  That converts the proportioning valve to a simple differential pressure valve with no proportioning effect.  Then plumb the aftermarket proportioning valve downstream of the Granada valve in the steel line heading to the rear axle.

When I took off the big plug to remove the piston/and spring, I noticed that the plug has a hole in it with a rubber plug in it.  The plug will not hold pressure (obviously) when the brake pedal is depressed.  Have any of you seen this type of proportioning valve plug before? Do you have a suggestion for how to disable the proportioning function of the valve? Can I weld the hole in the big nut/plug closed?

Thank you very much for any advice you are able to give.